The instance variable is accessible only within the class that declares it.
@protected
The instance variable is accessible within the class that declares it and within classes that inherit it. All instance variables without an explicit scope directive have @protected scope.
@public
The instance variable is accessible everywhere.
@package
Using the modern runtime, an @package instance variable has @public scope inside the executable
image that implements the class, but acts like @private outside.
The @package scope for Objective-C instance variables is analogous to private_extern for
C variables and functions. Any code outside the class implementation’s image that tries to use the instance variable gets a link error.
This scope is most useful for instance variables in framework classes, where @private may be too restrictive but @protected or @public too
permissive.